1. Field of the Invention
This invention relates generally to the preparation of esters of carboxylic acids and more particulary to a process for preparing glycol monoesters of sterically hindered carboxylic acids in very high selectivity.
2. Description of the Prior Art
British Pat. No. 1,119,897 relates to a process for reaction of an alkylene oxide with a sterically hindered carboxylic acid, which acid selectively favors the formation of the glycol monoester while inhibiting the formation of undesirable by-products. Catalyst disclosed for this purpose are alkali metal hydroxides, alkaline earth metal hydroxides, salts of strong basis, sodium metal, sodium alkoxides (such as sodium methylates) and, under certain conditions, mineral acids, such as sulfuric acid. Sodium hydroxide is said to be preferred.
While this method permits the efficient production of such glycol monoesters, it has not been possible heretofore to form the glycol monoesters in carboxylic acid conversions greater than about 90% without the formation of undesirably large amounts of by-product such as the di(carboxylic acid) ester of the glycol. For example, in the first example of British Pat. No. 1,119,897, the monoester was produced in high yield (81.7%), except that 7% of the unreacted neodecanoic acid and 11.1% diester and 2-mole ethoxylate impurities were present. Also, in the second example as the neo acid conversion approaches 100%, the concentration of the monoester rapidly decrease from 94.5% to 35%, with substantial increases in both diester and the 2-mole ethoxylated products effected.
Continued addition of ethylene oxide yielded higher ethoxylated products. The same effect, the very sudden and rapid decrease in a monoester concentration, as the conversion of the neo acid approach to 100%, is also noted in Example 3. These examples illustrate the difficulties in obtaining a high purity monoester product using the prior art process. The exact amount of ethylene oxide added to the reaction is required by the prior art patent to be carefully monitored in order to obtain monoester in high yield. Also, terminating the reaction before complete neo acid conversion is achieved is required in order to obtain monoester in an acceptable yield. This results in costly separation schemes in order to remove the unreacted neo acid from the desired monoester product.
In addition, the use of potassium or sodium hydroxides as catalyst has the disadvantage of forming water of reaction arising as a result of the neutralization of the neo acid reactant and the alkali metal hydroxide catalyst. Since water is an undesired contaminant, which can lead to by-product formations, the control of this water content is a process disadvantage.